<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <script>
        window.onload = function() {
            console.log('页面加载完成');
            var $button = document.querySelector('button');
            $button.onclick = function() {
                alert('哈哈哈哈');
            }
        }
    </script>
</head>

<body>
    <button>点我</button>
</body>

</html>